The present disclosure relates to a method for controlling exhaust gas aftertreatment in an exhaust gas aftertreatment system having at least one nitrogen oxide storage catalyst and at least one catalyst for selective catalytic reduction.
Nitrogen oxide storage catalysts (also referred to as lean NOx traps, LNT) may be used for the temporary adsorption of nitrogen oxides (NOx) from the exhaust gas of combustion engines. In addition, they perform their functions of oxidative aftertreatment of carbon monoxide (CO) and hydrocarbons (HC). Nitrogen oxides formed in lean-mixture operation of a combustion engine can be stored in an LNT; for this purpose, the LNT oxidizes the nitric oxide (NO) contained in the lean exhaust gas to nitrogen dioxide (NO2) and then stores it in the form of nitrates. Barium oxide and/or other oxides, for example, are used as adsorption agents, which are incorporated into the coating of the LNT.
Once the storage capacity of the LNT is exhausted, LNT regeneration is desired. During a regeneration event (purge), rich exhaust gas conditions may be provided for a few seconds, e.g. by operating the combustion engine with a rich, i.e. substoichiometric, fuel/air mixture; during this process, the stored nitrogen oxides are desorbed again and are reduced to nitrogen over catalytically active components of the LNT with the aid of the rich exhaust gas constituents (CO, HC). In addition to a purge effected purely for regeneration, the LNT is also regenerated if the exhaust gas becomes richer owing, for example, to rich-mixture operation of the combustion engine due to a power demand.
In the LNT, the stored nitrates furthermore react with molecular hydrogen, which is formed under rich exhaust gas conditions owing to incomplete combustion of the fuel and also owing to reactions in the LNT, as a result of which ammonia is also produced during a regeneration. It is possible to make use of this ammonia by storing it downstream in a passive catalyst for selective catalytic reduction (SCR). In the SCR, the stored ammonia is used to reduce nitrogen oxides to nitrogen under lean exhaust gas conditions. To enable the SCR catalyst to have a high storage capacity, it is advantageously installed sufficiently far downstream to ensure that optimum operating temperatures are obtained. The corresponding temperature range is a function of the specific SCR coating and is known to a person skilled in the art. Herein, upstream and downstream may be described relative to an exhaust gas flow from an engine, wherein upstream refers to components closer to the engine than downstream. As such, exhaust gas contacts upstream components before downstream components.
Among the factors limiting the storage capacity of an LNT is the temperature of the exhaust gas. Modern LNTs can store nitrogen oxides with different degrees of efficiency in a temperature range of 250-550° C. The storage capacity can furthermore be limited by the space velocity of the exhaust gas. When the combustion engine is operated under a high load, e.g. during an acceleration event, high exhaust gas temperatures and velocities are brought about, and these exceed the technological limits of the LNT. In this case, there can be a “nitrogen oxide breakthrough” since the nitrogen oxides cannot be stored and escape from the LNT. This may lead to NOx slip, which includes emitting NOx from the vehicle to an ambient atmosphere outside the vehicle.
Previous solutions include injecting reductant into an exhaust system during high temperature engine operating parameters. However, the inventors have found issues with these applications. As one example, reductant injection systems rely on complex control schemes and valves to implement the desired injection volumes, injection timings, and the like. They often demand routine fill-up of a reductant reservoir, which may be cumbersome to a vehicle operator. Furthermore, these reductant systems are expensive to manufacture and present packaging restraints.
In one example, the issues described above may be addressed by a method comprising adjusting an air/fuel ratio of a combustion engine, the engine comprising an exhaust tract connected fluidly to the combustion engine, two or more exhaust gas aftertreatment devices arranged in the exhaust tract comprising at least one nitrogen oxide storage catalyst, at least one catalyst for selective catalytic reduction arranged downstream of the first nitrogen oxide storage catalyst, at least one lambda probe, at least one temperature sensor, and a control unit, operating a combustion engine at low load during a first condition where a torque request is less than a threshold torque request, changing to an operating state with a high load during a second condition where the torque request is greater than the threshold torque request, adjusting the air/fuel ratio from more rich to less rich during the second condition in response to an ammonia load stored in the selective catalytic reduction reaching an upper threshold ammonia load, adjusting the air/fuel ratio from less rich to more rich during the second condition in response to the ammonia load stored in the selective catalytic reduction reaching a lower threshold ammonia load, and adjusting the air/fuel ratio from rich to lean in when switching from the second condition to the first condition in response to the torque request being less than the threshold request. In this way, fuel economy may increase and NOx conversion into NH3 may be increasingly utilized.
In one example, the emission of nitrogen oxides can be controlled under all operating conditions of the combustion engine. The high exhaust gas temperature which occurs at a high load is detected and transmitted to the control unit by the temperature sensor, which is advantageously arranged in the region of the first LNT. A large torque demand is furthermore detected and transmitted to the control unit in a manner known to a person skilled in the art. The control unit then triggers rich-mixture operation of the combustion engine if it is not already in effect. Under these conditions, ammonia is formed by the reaction of hydrogen with nitrogen oxides over catalytically active constituents of the LNT as soon as previously stored oxygen has been removed from the LNT. Downstream, the gaseous ammonia together with the exhaust gas flow from the LNT enters the SCR, where it is stored. When the storage capacity of the SCR is exhausted (e.g., full of ammonia), the combustion engine is temporarily operated under lean conditions. In this case, nitrogen oxides which are not stored in the LNT are transferred into the SCR with the exhaust gas flow and are reduced there to nitrogen by selective catalytic reduction using the stored ammonia. Once the stored ammonia has been consumed, the combustion engine is again operated in a rich mode. Once the high-load phase is past, the combustion engine is again operated in a conventional mode under lean conditions, in which nitrogen oxide is stored in the LNT, which is regenerated by brief purges. It is clear to a person skilled in the art here how a high and a low load of a combustion engine are defined. Herein, high load refers to a torque demand being greater than a threshold torque demand. An exhaust gas temperature is greater than a threshold temperature (e.g., 550° C.) when the engine load is high and NOx may leak from the LNT. As such, torque demands lower than the threshold torque demand may correspond to exhaust gas temperatures and conditions where the LNT may capture and store NOx without adjusting combustion air/fuel ratios.
In one embodiment, the combustion engine changes repeatedly from a rich to a lean combustion mode if the upper threshold value of the ammonia reserve stored in the SCR is reached, and from a lean to a rich combustion mode if the lower threshold value of the ammonia reserve stored in the SCR is reached. The upper threshold value of the ammonia reserve may be determined by measuring an ammonia slip through the SCR via a suitable sensor. In this way, the nitrogen oxide emissions can advantageously be controlled for as long as the high-load phase lasts.
Additionally or alternatively, some embodiments comprise a first and a second LNT. As a particular preference, the second LNT is arranged downstream of the first LNT in the flow direction of the exhaust gas. The arrangement of the second LNT is advantageous because it is subject to lower temperatures than the first LNT owing to the fact that it is further away from the combustion engine than the first LNT. It can therefore store nitrogen oxides which have not been stored in the first LNT or have escaped therefrom. The regeneration of the second LNT can furthermore take place in an effective manner by virtue of the substoichiometric conditions, by means of which the rich exhaust gas provided for the reduction of the first LNT is additionally enriched by ammonia and hydrogen from the first LNT and contains even less oxygen than the rich exhaust gas flowing directly out of the combustion engine.
The reaching of the upper threshold value of the ammonia reserve stored in the SCR is determined on the basis of a model. A self-ignition combustion engine may be used as the combustion engine. The engine may have any number of cylinders and geometries without departing from the scope of the present disclosure. As such, the engine may be an I-4 engine, a V-6 engine, a W-16 engine, and the like.
A second aspect of the present disclosure relates to an arrangement for carrying out a method according to the present disclosure, comprising a combustion engine, an exhaust tract connected fluidically to the combustion engine, an exhaust gas aftertreatment device arranged in the exhaust tract and comprising at least one first LNT, at least one SCR arranged downstream thereof, at least one lambda probe, at least one temperature sensor, at least one nitrogen oxide sensor, and a control unit. The arrangement comprises a second LNT, which is arranged downstream of the first LNT. In other words, the second LNT is arranged between the first LNT and the SCR. In one example, the first LNT is a close-coupled LNT, where a distance between outlets of the engine and the first LNT is minimized.
In some examples, there may be a first LNT and a second LNT, with no SCR located in the exhaust system. In the LNTs, the stored nitrates furthermore react with molecular hydrogen, which is formed under rich exhaust gas conditions owing to incomplete combustion of the fuel and also owing to reactions in the LNT, as a result of which ammonia can also be produced during a regeneration. It is possible to make use of this ammonia to further reduce the nitrogen oxide concentration in the exhaust gas in the second LNT arranged downstream of the first LNT.
In such an example, a method may reduce emission of nitrogen oxides under all operating conditions of the combustion engine. The high exhaust gas temperature which arises during a high load is detected by the temperature sensor, which is advantageously arranged in the region of the first LNT, and is transmitted to the control unit or determined by a stored temperature model. A high torque demand is furthermore also detected in a manner known to a person skilled in the art and transmitted to the control unit. This may be determined via a crankshaft sensor, a pedal position sensor, throttle position sensor, and the like. The control unit then initiates rich-mixture operation of the combustion engine unless it is already taking place. The first LNT no longer acts as a storage catalyst under these conditions but immediately converts the nitrogen oxides present in the exhaust gas to nitrogen with the aid of the reducing agents (carbon monoxide and hydrocarbons) likewise present in the exhaust gas. In this way, nitrogen oxides are advantageously removed from the exhaust gas emerging from the combustion engine under the conditions of a high load, while the nitrogen oxide storage efficiency of the LNT is greatly reduced owing to the gas temperature and the space velocity.
The rich-mixture component in the exhaust gas can furthermore be set in such a way that ammonia is formed under these conditions over the catalytically active constituents of the first LNT through the reaction of hydrogen with nitrogen oxides as soon as previously stored oxygen has been removed from the first LNT. This ammonia can be used downstream to further reduce the nitrogen oxides with the aid of a second LNT.
Once the phase of a high load is past, the combustion engine is once again operated in a conventional mode under lean conditions, in which nitrogen oxide is stored in the first LNT, which is regenerated by brief purges.
The phase of rich-mixture operation can furthermore also be ended if this is necessary to protect components, e.g. from excessive temperatures.
Step S3 of the method according to the present disclosure is preferably carried out if a predetermined threshold value of the temperature in the first LNT is reached. This threshold value is advantageously approximately at the temperature above which the LNT can no longer store nitrogen oxides efficiently. This value may be greater than 550° C.
The arrangement of the second LNT is desired because, owing to its being further away from the combustion engine than the first LNT, it is exposed to lower temperatures than the first LNT. In superstoichiometric exhaust gas conditions, it can therefore store nitrogen oxides which have not been stored in the first LNT or have escaped therefrom. The regeneration of the second LNT can furthermore take place in an effective manner by virtue of the substoichiometric conditions, by means of which the rich exhaust gas provided for the reduction of the first LNT is additionally enriched by ammonia and hydrogen from the first LNT and contains even less oxygen than the rich exhaust gas flowing directly out of the combustion engine. It is therefore particularly preferred if, in the method according to the present disclosure, the first nitrogen oxide storage catalyst is operated in such a way that it produces ammonia, which can be used in the second nitrogen oxide storage catalyst for the further reduction of nitrogen oxides.
A second aspect of the present disclosure relates to an arrangement which is designed for carrying out a method according to the present disclosure, comprising a combustion engine, an exhaust tract connected fluidically to the combustion engine, at least one first nitrogen oxide storage catalyst, at least one lambda probe, at least one temperature sensor and a control unit.
There is a particular preference for an embodiment of the arrangement in which the first nitrogen oxide storage catalyst is arranged in such spatial proximity to the combustion engine that exhaust gas temperatures which occur under high load prevent effective storage of nitrogen oxides, and the second nitrogen oxide storage catalyst is arranged at such a spatial distance from the combustion engine that effective storage of nitrogen oxides is possible even at exhaust gas temperatures which occur under high load.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.